It is known to operate metal halide high-pressure discharge lamps with accessory or operating apparatus supplying alternating current at frequencies in the ultrasonic range.
U.S. Pat. No. 4,170,746, Davenport, describes a process in which high-pressure discharge lamps are operated with alternating current at a frequency in a narrow range between 20 and 50 kHz, and in which no acoustic resonance results in the discharge medium when the lamp is operated. Acoustic resonance, as used in this specification, means standing acoustic waves. Acoustic waves within the discharge medium are caused by periodic changes of the instantaneous electrical energy fed via the lamp electrodes into the discharge medium. The instantaneous electrical energy varies at twice the alternating frequency of voltage, or current, respectively, supplied to the lamp. Experimentally determined resonance-free frequency bands are described in that patent.
When discharge lamps are operated in horizontal position, that is, in such position that the arc extends horizontally, the arc has a tendency to bend due to convection. The fill components within the discharge vessel which are in gas form or vapor form are bent upwardly, so that the discharge arc will have an upwardly bowed form. If this lamp is coupled to an optical system, the bending of the discharge arc interferes with optically forming an image of the light source and does not yield a symmetrical illumination pattern. Bowing or bending of the discharge arc also increases the thermal loading of the upper wall of the discharge vessel, decreasing the lifetime of the lamp.
It has been proposed to counteract the tendency of the arc to bend, see U.S. application Ser. No. 945,559, of Sep. 25, 1978, Johnson, to which published British Patent 2,034,107 corresponds, to provide a vertically directed homogeneous magnetic field which applies a downwardly directed force on the plasma in the discharge arc. The magnetic field is generated by an electromagnet which surrounds the discharge vessel in half-moon or semi-circular shape. It is expensive and difficult to secure and hold such an electromagnet.
It has been found that when metal halide discharge lamps are operated with electronic accessory apparatus providing higher frequencies, that is, higher than 20 kHz, there are narrow frequency ranges specific to lamps in which the discharge arc does not bend or bow. U.S. Pat. No. 5,121,034, Allen et al, discloses narrow frequency ranges, which were experimentally determined for low power metal halide high-pressure discharge lamps suitable for use in automotive applications.
The frequency regions disclosed in this patent are so selected that acoustic resonances occur within the discharge medium which reduce or even eliminate the bending of the discharge arc by convection. According to this patent, the operating frequency should be modulated in order to broaden the frequency range in which stable lamp operation with a straight arc is possible and, in this manner, to compensate for tolerances which arise in manufacture of the discharge vessel and which occur in the accessory apparatus. Modulating the operating frequency substantially complicates the circuitry.